HR: 10:50h
AN: GP42A-03    [Abstracts]
TI: Transitional Evolution of the Geomagnetic Field Recorded in Long Volcanic Sequences: Insights From Icelandic and Hawaiian Lavas
AU: Lau, J K
AF: SOEST-HIGP 1680 East West Rd, University of Hawaii at Manoa, Honolulu, HI 96822, United States
AU: * Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: SOEST-HIGP 1680 East West Rd, University of Hawaii at Manoa, Honolulu, HI 96822, United States
AU: Valet, J
EM: valet@ipgp.jussieu.fr
AF: Laboratoire de geomagnetisme et paleomagnetisme, Institut de physique du Globe de Paris, 4, place Jussieu, Paris, 75252, France
AB: We have studied the short-term evolution of the geomagnetic field recorded by long volcanic sequences in Iceland and Hawaii. The eastern Icelandic sequences correspond to 45 and 49 individual lava flows sampled and range in age from 12.9 to 10.21 Ma as reported by Watkins and Walker [Watkins and Walker, 1977. Am J. Sci. 277, 513-584] and are labeled as profiles C and D. Also sampled were long volcanic sequences in the Waianae, Koolau (island of O"ahu) and Mauna Loa (Big Island of Hawaii) volcanoes. We have studied in detail the directional characteristics of three successive reversals, the Gilbert-Gauss, the Lower and the Upper Mammoth reversals recorded by the Waianae lavas. These results confirm that large oscillations of directions precede or follow the reversals, which shows waveforms typical of paleosecular variation with their amplitude being considerably amplified by the decrease of the dipole. There is no apparent preferred location for the virtual geomagnetic poles (VGP). In addition to the directional analyses, determinations of absolute paleointensity were attempted on more than 540 samples, which document the field variations surrounding the Lower Mammoth transition. A period of a weak field dominated before the reversal; then the transition was initiated by a transit from normal to reverse polarity followed by a short restoration of field intensity in reverse polarity. A second episode of a very weak field was accompanied by a return to positive inclinations before reaching the reverse polarity. The very strong and apparent rapid recovery of the dipole following completion of the reversal culminated at a value of 16 x 10²² Am² similar to field intensities reported for the other detailed volcanic records of reversals studied so far. The asymmetry between the pre- and the post-reversal phases appears as a dominant characteristic and indicates the importance of field regeneration to initiate a new stable polarity interval. In addition we have obtained reversal records from the Koolau Volcano corresponding to Cryptochron C2r.2r-1 (ca. 2.514 ±0.030 Ma) and of the Laschamp and Pringle Falls excursions recorded by the Mauna Loa volcano (Big Island of Hawaii). The Icelandic as well as the Hawaiian transitional VGPs lie within the longitudinal bands of America, central Africa, Western Europe and eastern Asia. This distribution does not support the hypothesis of a direct link with heterogeneities of the lower mantle underneath the Americas and eastern Asia. Clusters of VGPs are observed in most records at various geographical locations without preference for specific longitudes, which in the present case most likely seems to result from intense volcanism during short time periods rather than from specific transitional states.
DE: 1513 Geomagnetic excursions
DE: 1520 Magnetostratigraphy
DE: 1521 Paleointensity
DE: 1522 Paleomagnetic secular variation
DE: 1535 Reversals: process, timescale, magnetostratigraphy
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting